A treaty signed in 1994 by the US and Britain could pull both countries into a war to protect Ukraine if Putin’s troops intervene.

The Budapest Memorandum was signed by Bill Clinton, John Major, Boris Yeltsin and Leonid Kuchma – the then-rulers of the USA, UK, Russia and Ukraine – as part of the denuclearization of former Soviet republics after the dissolution of the Soviet Union

Technically it means that if Russia has invaded Ukraine then it would be difficult for the US and Britain to avoid going to war.

Sir Tony Brenton, who served as British Ambassador from 2004 to 2008, said that war could be an option ‘if we do conclude the [Budapest] Memorandum is legally binding.’

It promises to protect Ukraine’s borders, in return for Ukraine giving up its nuclear weapons.

Today Kiev has demanded the agreement is activated after insisting their borders had been violated.

In response Mr Brenton said in a BBC radio interview: ‘If indeed this is a Russian invasion of Crimea and if we do conclude the [Budapest] Memorandum is legally binding then it’s very difficult to avoid the conclusion that we’re going to go to war with Russia’.